[Regulation Effects of Humus Active Components on Soil Cadmium Availability and Critical Threshold for Rice Safety].

Organic materials containing humic acids (HAs) play important roles in regulating the bioavailability of cadmium (Cd) in soils and thus its accumulation in crops. The effects of the two active components of HAs, humic acid (HA) and fulvic acid (FA), in organic materials and their different ratios (HA/FA) on Cd uptake and accumulation in rice were investigated using a field plot experiment, and their relationships with the Cd fractions and availability in paddy soil as influenced by the use of these organic materials were analyzed in combination with the fractionation method of chemical continuous extraction. The results showed that the effects of HAs on Cd availability in soil and Cd accumulation in rice grains were controlled by the ratios of the active components in the organic materials. The treatments with an HA/FA ratio ≥ 4/6 had a passivating effect on soil Cd, resulting in a significant reduction in Cd availability. Compared with that in the control without the application of HAs (CK), rice grain Cd concentration was reduced by 15.2%-33.3%, whereas those with an HA/FA ratio ≤ 2/8 activated Cd in soil, and the available Cd content was significantly increased. Compared with that in CK, rice grain Cd concentration was increased by 24.2%-42.4%. The ratios of HA/FA in HAs affected the morphological transformation of soil Cd. Compared with the CK treatment, the treatments with ratios of HA/FA ≥ 4/6 promoted the transformation of soil Cd from the exchangeable form (EX-Cd) with high activity to the carbonate bound form (CA-Cd) and Fe and Mn oxide-bound forms (FM-Cd) with low activity, whereas those with ratios of HA/FA ≤ 2/8 showed the opposite effects. The effects of HA and FA on soil pH and available sulfur concentration differed. Soil pH had a significant positive correlation with HA addition but a negative correlation with FA addition, and soil available sulfur content had a significant positive correlation with FA addition at the rice tillering stage. Therefore, to ensure the quality and safety of rice, organic materials with an HA/FA ratio ≥ 4/6 should be selected. The results provided a scientific basis for the directed utilization of organic materials containing HAs.

[Adsorption Characteristics of Arsenic and Cadmium by FeMnNi-LDH Composite Modified by Fulvic Acid and Its Mechanisms].

Toxic As(Ⅲ) and Cd(Ⅱ) ions in water can be transferred and enriched into human bodies through the food chain, causing serious health damage at excessive levels. In this study, fulvic acid (FA) was selected as the modifier of iron-manganese-nickel layered double hydroxide (FeMnNi-LDH), and a stable layered composite (FA@FeMnNi-LDH) was prepared using the co-precipitation method, which could adsorb As(Ⅲ) anions and Cd(Ⅱ) cations simultaneously, especially with the higher adsorption capacity of the cation Cd(Ⅱ). Its structure was characterized by XRD, TEM, FT-IR, and XPS, and the adsorption capacity and mechanisms of As(Ⅲ) and Cd(Ⅱ) in water by the composite were also investigated. The results showed that with typical characteristic peaks of layered double hydroxides, the synthesized composite possessed a stable structure, maximum FA loading capacity, and optimal adsorption performance. The adsorption kinetics of As(Ⅲ) and Cd(Ⅱ) conformed to the pseudo-second-order kinetic model, and the adsorption isotherms well-followed the Langmuir model, with the maximum adsorption capacity at 25℃ being 249.60 mg·g-1 for As(Ⅲ) and 156.50 mg·g-1 for Cd(Ⅱ), respectively. The composite exhibited a good adsorption performance on As(Ⅲ) and Cd(Ⅱ) in the range of pH 2-7 and pH 4-7, respectively. The competitive adsorption effect of co-existed anions on As(Ⅲ) showed a sequence of PO43->CO32->NO3-, and that of co-existed cations on Cd(Ⅱ) was Pb2+>Cu2+>K+. The adsorption capacity of As(Ⅲ) and Cd(Ⅱ) decreased with the increase in the concentration of competing ions. The main adsorption mechanism for As(Ⅲ) was ion-exchange occurring in the interlayers of LDH, and that for Cd(Ⅱ) was coordination complexation occurring with the loaded FA, respectively. In conclusion, the prepared FA@FeMnNi-LDH composite material posed a good application prospect for adsorption removal of As(Ⅲ) and Cd(Ⅱ) in water and their toxicity control.

[Screening and Evaluation of Methods for Determining Available Lead (Pb) and Cadmium (Cd) in Farmland Soil].

The accumulation of heavy metals in crops is largely dependent on the availability of heavy metals in soils. Due to the differences of soil types and pollution characteristics, there is no widely recognized method for the determination of the bio-available heavy metals in soils such as Pb and Cd. In order to screen and establish suitable methods, the extractable abilities of five extractants (CaCl2, NH4OAc, HCl, EDTA, and DTPA) and Gradient Diffusion Film Technology (DGT) on four typical farmland soils with very different properties in Chongqing (acid purple soil, neutral purple soil, calcareous yellow soil and calcareous purple soil) were systematically compared. Simultaneously, pot experiments were carried out with Lolium perenne L. as an indicator plant to explore the relationship between the extractable soil Pb and Cd and their accumulation in plants. The feasibility of the tested methods were evaluated based on the extractability and co-relationships. The results showed that the extractability of the tested extractants for soil Pb and Cd varied a lot. HCl showed highest extractability on Pb in acid purple soil and calcareous yellow soil, while EDTA did the best in neutral purple soil and calcareous purple soil; HCl showed best extractability in all soils except calcareous purple soil, where EDTA was the best. Considering the correlation between the extraction amount and uptake by plant, all the extraction methods could be applied for the evaluation of the bioavailability of soils Pb and Cd except CaCl2 for Pb due to its weak extractability for Pb for a specific soil type. For the comparison of heavy bioavailability in different soil types, EDTA-extractable Pb and DGT-extractable Cd were recommended due to their well co-relationships between extractable amount in soil and uptake by plant (Lolium perenne L.) as indicated by the correlation coefficients of 0.941 and 0.919, respectively. HCl was relatively suitable as Cd extractant compared to others if DGT method could not be used.

[Safe Utilization of Paddy Soils Lightly Polluted with Cadmium in Western Chongqing].

The western Chongqing region is the main grain-producing area in Chongqing. This region's soils are characterized by light-to-moderate cadmium (Cd) pollution. Two types of typical paddy soils in this area (acidic and calcareous purple soils) were selected for the development of safe rice production techniques using in situ field remediation experiments. These involved a low-Cd-accumulating rice variety (Changliangyou 772) grown either alone or in combination with heavy metal passivators (silicon-calcium fertilizer, iron powder, biochar, and straw organic fertilizer). The results showed that:① all of the passivators except for the Fe powder increased rice yields from both types of soils. Straw organic fertilizer showed the best results for the acidic purple paddy soil, with a rice-yield increase of 47.43%, while biochar performed best for the calcareous paddy soil, increasing yields by 23.95%; ② The low-accumulation rice variety alone could not meet the requirements of safe rice production in the acid purple paddy soil (pH=4.75); however, combined the with passivators (with the exception of the silicon-calcium fertilizer), Cd content in rice grains was reduced by 14.81%-54.88% to within the national safe food standard for rice (0.2 mg·kg-1, GB 2762-2017). The Cd content of rice grains varied between 0.012 and 0.030 mg·kg-1 in the calcareous purple paddy soil (pH=7.77), under various treatments, which was far lower than the safety standard. The application of passivators (with the exception of biomass charcoal) further reduced the Cd content of rice by 26.67%-59.00% nevertheless; ③ The use of passivators altered Cd transportation and distribution in the rice plant. Taking the acidic soil as an example, silicon-calcium fertilizer inhibited the transport of Cd from the stems to the rice grains, Fe powder and biochar reduced the enrichment of Cd in roots and inhibited the transport of Cd from the stems to the rice grains, and straw organic fertilizer inhibited the transport of Cd from the roots to the stems; ④ The application of passivators promoted the transformation of soil Cd from labile forms into residual form, reduced the bioavailability of Cd in the soil, and, thus, reduced the accumulation of Cd in the rice plants. In the acidic soil, biochar showed the best effect, while straw organic fertilizer performed best in calcareous soils. ⑤ Silicon-calcium fertilizer and straw organic fertilizer significantly increased the pH and organic matter content of acid soils. Consequently, the soil available Cd content decreased by 39.45% and 34.69%, respectively, while no such effects were observed for the calcareous soil.